Add Guardian V2 analytics events (#42144)

## What changed

- Emit `codex_guardian_v2_classification` events with the outcome, risk
  level, duration, model, and thread attribution.
- Emit `codex_guardian_v2_fast_decision` events when Guardian V2 approves an
  action without a full review.
- Add `guardian_v2_enabled` to turn analytics, accounting for review scope
  and model requirements.

## Testing

- Extend the Guardian V2 app-server coverage to verify event payloads,
  attribution, enablement state, and scoped event emission.

GitOrigin-RevId: 0805975f8e8f9d954aa1058ea8c60545a090ae74

codex-rs/ext/extension-api/src/lib.rs

@@ -47,6 +47,7 @@ pub use contributors::ApprovalReviewInput;
 pub use contributors::ConfigContributor;
 pub use contributors::ContextContributor;
 pub use contributors::ExtensionFuture;
+pub use contributors::GuardianV2Enabled;
 pub use contributors::McpServerContribution;
 pub use contributors::McpServerContributionContext;
 pub use contributors::McpServerContributor;